These experiments investigate interactions between cerebromicrovascular endothelial cells (EC) which constitute the blood-brain barrier (BBB) and peripheral blood cells and/or components. Adhesive interactions between circulating monocytes and EC were examined in situ and in vitro. These experiments utilized (10-16 wk) spontaneously hypertensive rats (SHR), stroke-prone SHR (SHR~SP), normotensive Wistar-Kyoto (WKY), Sprague- Dawley (SD) and Fisher 344 (F344) rats, and old (2-3 yrs) SD and F344 rats. The in vivo experiments assessed the number of perivascular macrophages (ED2-positive cells) in blood vessels of perfusion-fixed frozen brain sections. The data demonstrated increased numbers of perivascular macrophages in cerebral intraparenchymal vasculature of hypertensive and aged rats. Experiments also demonstrate that LPS-induced (i.v. and i.c.v.) release of von Willebrand factor (vWF) in SHR rats was significantly greater (p<0.05) than in WKY rats. Stroke risk factors such as hypertension may predispose endothelium to be more responsive to agonist-stimulated secretion of vWF (which is important in homeostasis and thrombosis). In vitro experiments using cultured cerebromicrovascular EC derived from SHR and WKY rats demonstrated increased monocyte adhesivity to cytokine- or LPS-treated EC. The level of up-regulation of monocyte adhesion to SHR EC was significantly greater than to similarly treated WKY EC. The results suggest that stroke risk factors hypertension and advanced age are associated with: (1) an increased adhesion of monocytes to endothelium (in vivo) and EC monolayers (in vitro); (2) a hyper-responsive state of monocytes; and (3) an increased tendency of endothelium to convert to a procoagulant surface. All the above findings implicate factors such as cytokines and LPS in disorders involving recruitment, attachment and/or transvascular migration of blood cells at sites of inflammatory responses. The data indicate how advanced age and hypertension may act as risk factors for stroke (i.e., increase the likelihood of interactions between monocytes and endothelium leading to local thrombosis or hemorrhage).